The present invention relates to a steel member such as tube, plate, bar, wire or the like plated with a Pb-Sn alloy to have an increased corrosion resistance, suitable for use as the material of a fuel tank for gasoline, alcoholic fuel or the like, as well as to a method of producing the same.
Hitherto, steel material plated with Pb-Sn alloy has been used as materials of various parts such as automobile fuel tank, parts of radiator, chemical vessel and so forth, because of superior corrosion resistance, solderability and workability, as well as moderate cost.
However, Pb as the plating metal and the steel can hardly react with each other to make the formation of the alloy layer therebetween difficult. In addition, since the Pb itself is liable to be oxidized, it is difficult to adjust the amount of deposition of the plating alloy, often resulting in a problem of formation of pin holes. It is also to be pointed out that the Pb-Sn alloy which is inherently soft tends to be damaged during handling or the press work to develop scratches or pin holes. This also promote the generation of rust particularly when the material is used under a corrosive environment.
Recently, due to the shortage of crude oil resources, there is an increasing need for alcoholic automobile fuel in which alcohol such as methyl alcohol, ethyl alcohol, methyl tertiary butyl ether or the like (generally referred to as "gasohol" or even pure alcohol fuel. This gives a rise to a demand for steel sheet plated with Pb-Sn alloy having higher corrosion resistance and capable of suppressing the formation of pin holes as compared with conventional steel sheet plated with Pb-Sn alloy.
Various proposals have been made to cope with this demand. For instance, it has been proposed to effect the coarseness of the surface of the substrate material through a heavy pickling for long time and at high temperature, thereby to increase the reactivity and mechanical bonding between the steel and the Pb-Sn plating alloy. It has also been proposed, in order to prevent the generation of pin holes due to inadequate roll squeezing for adjusting the amount of deposition of the plating alloy, to conduct the roll squeezing while covering the entire surface of the plating bath with a flux or palm oil, irrespective of whether the roll squeezing is effected by a pair of rolls to which the flux is deposited or by means of a high-pressure gas wiping.
Also, in the specification of U.S. Pat. No. 3,875,027 (corresponds to Japanese Patent Publication No. 29986/1976), as well as in Japanese Patent Publication No. 51426/1980 issued to the present inventors, proposed are new methods effective for preventing generation of pin holes in the Pb-Sn alloy plated steel sheet, in which the steel sheet is beforehand plated with Ni in advance to the plating with molten Pb-Sn alloy.
More specifically, in the method proposed in the specification of U.S. Pat. No. 3,875,027, the plating with Pb-Sn alloy (Terne plating) is effected after a plating with Ni to a thickness of 7.62 to 48.3.mu.. The aforesaid plated steel sheet is used mainly as the pipe material of automobile fuel tanks. In the method disclosed in Japanese Patent Publication No. 51426/1980, the plating with molten Pb-Sn alloy (Terne plating) is effected after having applied Ni plating to a thickness of 0.03 to 1.0.mu.. According to this method, it is possible to produce steel sheet plated with Pb-Sn alloy having a high corrosion resistance and reduced number of pin holes, even when the pickling before the plating with molten Pb-Sn alloy are simplified.
Another proposed method is to effect an undercoating treatment on the steel surface by forming a single plating layer of Zn, Sn, Cu or the like in advance to the plating with the molten Pb-Sn alloy, as in the case of the pretreatment with Ni stated before. The Zn and Sn in the undercoating layer, however, is liable to be re-dissolved in the plating bath of Pb-Sn alloy during the subsequent plating with Pb-Sn alloy. Also, the Cu does not show a good adhesion to steel so that sufficient effect for preventing the generation of pin holes cannot be obtained. The Ni has an appreciable effect on prevention of the generation of pin holes. However, in the conventional process, the Ni inconveniently forms fragile Ni-Sn layer such as Ni.sub.3 Sn.sub.2, Ni.sub.3 Sn.sub.4 phases or the like to reduce the adhesion between the Ni layer and the Pb-Sn alloy plating layer often resulting in a separation during press work, particularly when the plating dipping time is too long or when the cooling after the plating is effected too gradually.
As is well known, automobile fuel tanks are produced by subjecting the material to a considerably severe shaping or forming process, such as having a shape of 300 mm height including bulging work. It is, therefore, essential that the bonding strength between the steel sheet and the plating layer is sufficiently high to avoid separation of the latter during the forming work.
In general, in order to avoid separation of plating layer in the subsequent mechanical work of metal-plated products, it is necessary that the thickness of the plating layer is not excessively large.
In the case where the metal plating is effected on a steel sheet by employing a heating means as in the case of hot dip plating, heating diffusion or the like, the main plating layer is deposited usually through an intermediate alloy layer or layers. This alloy layer is formed of an intermetallic compound which is generally hard and brittle. For instance, in the case of hot dip plating with molten Zn, an intermetallic compound of Zn-Fe alloy system is formed, while, in the case of ordinary hot dip plating with molten Pb-Sn alloy, an intermetallic compound of Fe-Sn system is formed. It is well known that the bonding strength of the plating layer is seriously decreased if this alloy layer has a large thickness. Thus, a too large thickness of this alloy layer is not preferred also, in order to obtain high bonding strength of the plating layer.
This phenomenon is observed also in the case where the Pb-Sn alloy, which generally is soft and has a high lubricating effect, is used as the coating layer, as well as in the aforementioned case where an undercoating layer of Ni is formed to present the possibility of formation of an alloy of Ni and Sn on which the plating layer of Pb-Sn alloy is formed.
From this point of view, the product plated with Pb-Sn alloy shown in the specification of the U.S. Pat. No. 3,875,027 is mainly directed to steel piping subjected to bending or slight bending, so it cannot be used suitably as the material of automobile fuel tanks, because of a large susceptibility to separation of the plated layer during the mechanical forming work which is conducted under a severe condition.
The Japanese Patent Publication No. 51426/1980 proposes a technique which offers the following advantages.
(1) Shortening and simplification of pretreatment pickling.
(2) An alloy layer of Ni-Sn alloy is formed uniformly instead of the Fe-Sn alloy to reduce the formation of pin holes after the plating with molten Pb-Sn alloy.
(3) Even when the alloy layer is exposed during the control of the deposition amount, no rust is formed because the alloy layer is of Ni-Sn system, in contrast to the case where the alloy layer is of an Fe-Sn alloy.
(4) The Ni undercoating layer and the Pb-Sn alloy plating layer provide superposed effect on the prevention of generation of pin holes.
Thanks to the above-listed features, this method permits the production of corrosion-resistant product plated with molten Pb-Sn alloy, even when the pretreatment by pickling is conducted at a high speed and in a simplified manner.
These prior arts explained heretofore are merely the technique for producing products plated with Pb-Sn alloy, highly resistant to corrosion and having reduced number of pin holes. At the present stage, however, it is not considered that a technique has been established for the production of materials having a steel sheet base, a Ni undercoating layer and a plating layer of Pb-Sn alloy formed by hot dip plating, optimum for use as the material of automobile fuel tank, particularly suitable for the material of tank for fuel having alcoholic content.